1. Field of Invention
The present invention relates to a spindle motor having dynamic pressure bearing for hard disk drive (HDD) which stably retains lubricant in a bearing portion.
2. Description of Related Art
FIG. 4 shows a cross-sectional view of a conventional spindle motor for hard disk drive (HDD). In FIG. 4, a spindle motor 300 is composed of a motor base 1, a shaft 2, dynamic pressure grooves 3 and 4, a hub 5, a cylindrical sleeve 6, a thrust plate 7, a flange 8, a plurality of cores 9, a coil 10, a ring magnet 11, a yoke 12, a rotor 13, and a stator 14. xcex83 is an angle of a taper portion.
The stator 14 of the spindle motor 300 for HDD is composed of the motor base 1, the core 9 and the coil 10. The rotor 13 is rotatably placed in a position opposed to the stator 14, and is composed of the shaft 2, the hub 5, the ring magnet 11 and the yoke 12. The shaft 2 for the rotor 13 is rotatably positioned in the cylindrical sleeve 6 fixed at the center of the motor base 1.
The motor base 1 described above is made of aluminum or aluminum alloy. The plurality of cores 9 wound up with the coil 10 is fixed circularly around the sleeve 6. The shaft 2 is made of a stainless steel system material. The hub 5 for the rotor 13 has the ring magnet 11 and the yoke 12 placed in a position opposed to the core 9 for the stator 14. In addition, the peripheral portion of the hub 5 has a structure for connecting a hard disk (hereinafter referred to as HD; not shown) for recording data information.
The shaft 2 and the sleeve 6 compose a radial dynamic pressure bearing portion to bring dynamic pressure towards the radial direction in the cylindrical sleeve 6.
The dynamic pressure grooves 3 and 4 are formed inside of the cylindrical sleeve 6 contiguously opposed to the peripheral surface of the shaft 2 being inserted rotatably in the inside of the cylindrical sleeve 6. The dynamic pressure grooves 3 and 4 have herringbone shaped grooves formed in sideways.
The peripheral surface of the shaft 2 has a first shaft portion contiguously opposed to the dynamic pressure groove 3 and 4, and a second shaft portion sandwiched by two of the first shaft portion. The diameter of the first shaft portion is usually bigger than that of the second shaft portion.
In the cylindrical sleeve 6, lubricating oil having predetermined viscosity is filled in between the space inside surface of the sleeve 6 and the peripheral surface of the shaft 2. The lubricating oil flows in the space between the dynamic pressure grooves 3 and 4, and between two of the first shaft portions respectively. The lubricating oil to be filled in the radial dynamic pressure bearing portion and in the thrust dynamic pressure bearing portion (as explained below) also flows in each space of the dynamic pressure bearing.
In the radial dynamic pressure bearing portion described above, the dynamic pressure towards the radial direction occurs by the dynamic pressure grooves 3 and 4 of the cylindrical sleeve 6 and the lubricating oil when the shaft 2 rotates. The dynamic pressure towards the radial direction is putting a pressure force equally on the peripheral surface of the shaft 2.
The pressure force towards the periphery of the shaft 2 provides stable rotation of the shaft 2 in the cylindrical sleeve 6. It is obvious that the dynamic pressure grooves 3 and 4 can be formed circularly on the peripheral surface of the shaft 2 to make dynamic pressure towards the radial direction.
The thrust dynamic pressure bearing portion is composed of the flange 8 fixed at the bottom of the shaft 2, and the thrust plate 7 covering the bottom portion of the cylindrical sleeve 6.
The plane surface of the flange 8 is formed with herringbone shaped dynamic pressure grooves not shown. The flange 8 is a disciform shape and has a hole in the center thereof, and is made of copper system material. The bottom portion of the shaft 2 fits with the hole in the flange 8 and is bonded to become one piece with the flange 8.
The bottom portion of the inner surface of the cylindrical sleeve 6 has two step-shaped differences in concentric circle of which center crosses the rotational axis. The deeper difference fits with the flange 8, which becomes one piece with the shaft 2. Then the thrust plate 7 fits with the shallower difference to cover the inner bottom portion of the cylindrical sleeve 6.
Consequently, the shaft 2 is rotatably supported in the cylindrical sleeve 6 with the flange 8 and thrust plate 7. Before covering the bottom portion of the cylindrical sleeve 6, the lubricating oil is filled in the space between the shaft 2 and the flange 8 and the thrust plate 7, inside the cylindrical sleeve 6.
As a result, the bottom portion of the shaft 2 contacts with the thrust plate 7 when the shaft 2 is not rotating, and moves from the thrust plate 7 when the shaft 2 is rotating.
As to the thrust dynamic pressure bearing portion, the dynamic pressure towards the thrust direction occurs by the inner surface of the cylindrical sleeve 6 contiguously opposed to the flange 8, and two dynamic pressure grooves formed on the flange 8, and the thrust plate 7 and the lubricating oil when the shaft 2 rotates.
The dynamic pressure towards the thrust direction is putting a pressure force equally on the plane surface of the flange 8, which becomes one piece with the shaft 2. More specifically, the dynamic pressure balances a downward force to push down the rotor 13 by the dynamic pressure groove formed on the upper plane surface of the flange 8 and an upward force to push up the rotor 13 by the dynamic pressure groove formed on the lower plane surface of the flange 8. The balance of the upward and downward force rotatably holds the rotor 13 supported by the shaft 2 (the lubricating oil will stay in the space between the shaft 2 and the thrust plate 7 when the shaft 2 is rotating).
As described above, the lubricating oil having predetermined viscosity is filled in each dynamic pressure bearing portion to bring dynamic pressure towards the radial direction in the radial dynamic pressure bearing portion and towards the thrust direction in the thrust dynamic pressure bearing portion respectively for the spindle motor 300 for HDD.
The lubricating oil stays in each dynamic pressure bearing portion when the shaft 2 is not rotating. However, when the shaft 2 rotates, the lubricating oil moves to the open space in the upper portion of the inside of the cylindrical sleeve 6. A sealing described below is provided to the open space in the cylindrical sleeve 6 to prevent the lubricating oil from leaking out from the inside of the cylindrical sleeve 6, but it will be complicated and costly.
If the lubricating oil leaks out from inside of the cylindrical sleeve 6, it may stick on the surface of the HD mounted on the hub 5 of the rotor 13 to interfere recording and/or reproducing operation for the HD. Further, if the lubricating oil leaks out from the cylindrical sleeve 6, each dynamic pressure bearing portion runs out of the lubricating oil, and the dynamic pressure for radial direction and thrust direction can not be obtained properly. As a result, the rotor 13 can not rotate at the predetermined revolution, which would deteriorate the function of the spindle motor 300 for HDD.
Accordingly, the lubricating oil should be sealed properly. For example of sealing, a downward taper portion (having a taper angle xcex83) is provided at the upper portion of the inside of the sleeve 6 as shown in FIG. 4. A magnetic fluid not shown can also be used for sealing. Further, a labyrinth structure can be provided between the upper portion of the cylindrical sleeve 6 and the inner circumference of the rotor 13 to seal the lubricating oil. However, the sealing described above are too complicated and costly for the spindle motor 300.
Accordingly, in consideration of the above-mentioned problems of the related art, an object of the present invention is to provide a spindle motor having a radial dynamic pressure bearing portion and a thrust dynamic pressure bearing portion, the spindle motor including a shaft (2A, 2B) having a first taper surface (2Ad) on the periphery thereof for supporting a rotor for rotation relative to a stator, wherein the first taper portion has a taper angle xcex81 towards the upward direction of a rotational axis, a cylindrical sleeve (6A, 6B) for rotatably supporting the shaft and having a second taper portion (6Ad) opposed to the first taper portion via lubricating oil so as to form the radial dynamic pressure bearing portion, wherein the second taper portion has a taper angle xcex82 towards the downward direction of the rotational axis, wherein the first taper angle xcex81 is bigger than the second taper angle xcex82 (xcex81 greater than xcex82), and a thrust plate (7A, 7B) for rotatably supporting the shaft and for fixing the cylindrical sleeve with the stator so as to form the thrust dynamic pressure bearing portion.
Other object and further features of the present invention will be apparent from the following detailed description when lead-in conjunction with the accompanying drawings.